Nearly as soon as I read your comments, we decided not to go in Oliver's room any more (unless the crying escalates) and miraculously for the last three nights we had almost a full night sleep! He still makes his noises but they seem to be less intense. We found him a few times 180° turned in the bed, he must have worked out how not to get stuck halfway. He seems to be much happier to sleep on his tummy also, and must have learned to roll back (maybe while rotating!) as in the morning he's always on his back.
He seems to be back to his old self, when he wakes up in the morning he's happy to chat to himself and play with the soft toys he has in the bed.
So thanks so much for sharing and I hope it'll settle for your child too if you are still experiencing it!